Successive Phase Transitions in Crystals of K2MgF4-Type Structure

Abstract

The possible distorted phases determined by three types of octahedral tilts in the layered perovskite-like crystals of K2MgF4-structure are analysed. The group-theoretical analysis showed that θ-tilts are determined by irreducible representation τ7 and ϕ-tilts by τ3 and τ5 ones belonging to K13(Z) star of the initial G0 = D phase respectively. The experimental data concerning the phase transitions (PT's) in the crystals of (CnH2n+1NH3)2·BX4 with n = 1, 2, 3; B = Mn, Fe, Cd; X = Cl, and of A2BX4 with monoatomic A-ions are discussed. It is shown that the successive PT's are determined mainly by two two-component order parameters which correspond to τ3 and τ7. There exist the distorted phases which are distinguished only by signs of θ-tilts in the nearby layers. This sensitivity of the symmetry changes on the phase of tilts is an example of distortional polytypism (polydistortion) which was discussed earlier by the authors. It is also proved that PT in some A2BX4 crystals with monoatomic A-ions belong to the displacive type one. It is proposed that the instability of octahedral sheets in K2NiF4-type structure with respect to the tilts and the softness of the librational soft mode could be the main reason of the phase transitions in the whole family of the crystals.